The present invention relates generally to laser drilling, and more particularly, to a method for maintaining a workpiece in a focal plane of a laser drilling system.
Material ablation by pulsed light sources has been studied since the invention of the laser. Reports in 1982 of polymers having been etched by ultraviolet (UV) excimer laser radiation stimulated widespread investigations of the process for micromachining. Since then, scientific and industrial research in this field has proliferatedxe2x80x94mostly spurred by the remarkably small features that can be drilled, milled, and replicated through the use of lasers.
Ultrafast lasers generate intense laser pulses with durations from roughly 10xe2x88x9211 seconds (10 picoseconds) to 10xe2x88x9214 seconds (10 femtoseconds). Short pulse lasers generate intense laser pulses with durations from roughly 10xe2x88x9210 seconds (100 picoseconds) to 10xe2x88x9211 seconds (10 picoseconds). A wide variety of potential applications for ultrafast and short pulse lasers in medicine, chemistry, and communications are being developed and implemented. These lasers are also a useful tool for milling or drilling holes in a wide range of materials. Hole sizes as small as a few microns, even sub-microns, can readily be drilled. High aspect ratio holes can be drilled in hard materials, such as cooling channels in turbine blades, nozzles in ink-jet printers, or via holes in printed circuit boards.
Optical parallel processing of laser-milled holes is key to increasing the throughput of, and the profitability of laser micromachining. Beamsplitting devices such as diffractive optical elements are currently used in laser micromachining to divide a single beam into multiple beams to allow for parallel processing of the workpiece (i.e., material to be drilled).
Currently, one way to prevent a laser drilling system""s sub-beams from damaging the workpiece holder is to use a workpiece holder with a large recess behind the target area, such that the sub-beams pass through the workpiece holder after milling through the workpiece itself. Performing parallel laser drilling upon a flimsy workpiece presents a set of challenges related to keeping the flimsy workpiece surface in the focal plane. A workpiece holder with a single large recess behind the target area does not provide sufficient support to keep the flimsy workpiece in the focal plane when the foil is subject to recoil pressure due to laser ablation.
In order to perform precision laser drilling in a parallel process system, the workpiece surface must remain in the focal plane (where the laser beams are focused) of the laser drilling system throughout the laser drilling to enable the beams to drill workpiece geometries meeting precise specifications. However, the use of thin, flimsy workpieces (workpieces that bend and move outside the focal plane of the drilling laser beam when the workpiece is impacted with the beam(s)), which are required in some applications, such as inkjet nozzles, poses a challenge because the workpiece deforms during drilling and moves outside the focal plane of the laser system. This results in poor quality laser-drilled holes and an inability to meet required product specifications.
When a laser drilling system""s sub-beams are incident upon a flimsy workpiece, the kickback of debris causes significant recoil force upon the workpiece, causing the workpiece to deform and move outside the laser drilling system""s focal plane. If the sub-beams are out of focus when incident upon the workpiece, the result will be poor quality and misshapen holes that do not meet product specifications or obtain the desired benefits of precision laser micromachining. What is needed is a way to counteract workpiece deformation when using parallel process laser drilling on a flimsy workpiece.
One way to counteract the workpiece deformation is to reduce the atmospheric pressure in front of the workpiece. A reduction in atmospheric pressure exerts a force upon the workpiece that moves it toward the area of reduced atmospheric pressure. A sufficient reduction in atmospheric pressure in front of the workpiece counteracts the deformation of the workpiece caused by the recoil force.
In accordance with the present invention, a method is provided for maintaining a workpiece in a focal plane of a laser drilling system. The method includes: providing a workpiece holder that is adapted to releasably retain a workpiece on a planar surface thereof, the planar surface having a recess extending therein; positioning the workpiece onto a planar surface of a workpiece holder, such that the workpiece extends across the recess formed in the workpiece holder and an exposed surface of the workpiece aligns with a focal plane of a laser drilling system; projecting a laser beam from the laser drilling system onto the exposed surface of the workpiece, thereby forming an ablation on the exposed surface of the workpiece; and directing a flow of gas onto the exposed surface of the workpiece, substantially concurrent with the step of projecting a laser beam, such that the flow of gas substantially impinges on an area of the exposed surface that extends across the recess formed in the workpiece holder, thereby maintaining the exposed surface of the workpiece in the focal plane of the laser drilling system during the laser drilling operation.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.